1. Field of the Invention
This invention relates to process monitoring methods used for quality assurance of laser surface treatment of a metallic surface and, more particularly, a method for providing quality assurance of a laser shock peening (LSP) process by measuring a spectral intensity of light emitted by a laser generated plasma over its duration.
2. Description of Related Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses one or more radiation pulses from high power pulsed lasers to produce an intense shock wave at the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477 entitled "Laser Shock Processing"; and U.S. Pat. No. 5,131,957 entitled "Material Properties". Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by the instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface.
Laser peening has been utilized to create a compressively stressed protective layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421 entitled "Laser Peening System and Method". These methods typically employ a curtain of water flowed over the workpiece or some other method to provide a confining plasma medium. This medium enables the plasma to rapidly achieve shockwave pressures that produce the plastic deformation and associated residual stress patterns that constitute the LSP effect.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. Nos.: 5,756,965 entitled "On The Fly Laser Shock Peening"; 5,591,009 entitled "Laser shock peened gas turbine engine fan blade edges"; 5,569,018 entitled "Technique to prevent or divert cracks"; 5,531,570 entitled "Distortion control for laser shock peened gas turbine engine compressor blade edges"; 5,492,447 entitled "Laser shock peened rotor components for turbomachinery"; 5,674,329 entitled "Adhesive tape covered laser shock peening"; and 5,674,328 entitled "Dry tape covered laser shock peening", all of which are assigned to the present Assignee. These applications, as well as others, are in need of an efficient quality assurance evaluation method for production runs using laser shock peening.
Laser shock peening processes have been developed to simultaneously LSP pressure and suction sides of leading and trailing edges of fan and compressor airfoils and blades as disclosed in U.S. Pat. No. 5,591,009 entitled "Laser shock peened gas turbine engine fan blade edges" and U.S. Pat. No. 5,531,570 entitled "Distortion control for laser shock peened gas turbine engine compressor blade edges". Single-sided shot peened Almen strips are well known for use in the field of shot peening quality control, see U.S. Pat. No. 2,620,838. However, Almen strips are not designed to provide a measure of the effect of a single laser beam impact. The LSP process involves the use of high pulse energy, short pulse duration laser systems. The combination of high energy and short duration, as well as variations in the stability of the beam path (such as at a water/air interface), limit the usefulness of electronic measurement systems to verify the true (calibrated) energy being delivered to the component being processed.
One laser shock peening quality assurance technique that has been used is high cycle fatigue (HCF) testing of blades having leading edges which have been LSP'd and notched in the LSP'd area before testing. This method is destructive of the testpiece, fairly expensive and time consuming to carry out, and significantly slows production and the process of qualifying LSP'd components. HCF testing is a random sampling technique and is a poor statistical quality measurement. An improved quality assurance method of measurement and control of the LSP process that is a non-destructive evaluation (NDE), inexpensive, accurate, and quick is highly desirable. It is also desirable to have an NDE quality assurance method that is relatively inexpensive and sufficiently economical to be used directly on the actual workpiece instead of indirectly on a sacrificial sampling of workpieces. LSP is a process that, as any production technique, involves machinery and is time consuming and expensive. Therefore, any techniques that can reduce the amount or complexity of production machinery and/or production time are highly desirable.
The present invention measures instantaneous spectral intensity of light emitted by a laser generated plasma over the temporal duration of a single firing of a laser used in the laser shock peening process. The invention preferably measures instantaneous light intensity of the plasma through the analysis of an instantaneous optical spectrum associated with vaporized material using optical devices such as a streak camera available from Hamamatsu of Japan.